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Optimizing the increased oxide aperture relative to the laser dimensionsRelated Patent Categories: Coherent Light Generators, Particular Active Media, Semiconductor, Injection, Monolithic Integrated, Laser Array, With Vertical Output (surface Emission)Optimizing the increased oxide aperture relative to the laser dimensions description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070091959, Optimizing the increased oxide aperture relative to the laser dimensions. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention generally relates to vertical cavity surface emitting lasers (VCSEL) comprising a first reflector and a second reflector to define a resonator, wherein a laser active region is located between the first and the second reflector. Moreover, the VCSEL under consideration includes an aperture layer so as to control optical losses of higher-order transverse radiation modes, thereby providing a single transverse mode emission or emission with a reduced output wavelength range. [0002] VCSEL devices are considered an attractive alternative to conventional double-heterostructure laser diodes due to their small size and their potentiality of being formed in a substantially circular symmetry. Generally, VCSEL devices show a relatively low threshold current, a high modulation efficiency and, if designed so as to emit a substantially circular beam profile, allow to be coupled into optical fibers in a simple fashion. Additionally, the manufacture of VCSEL devices comes along with a parallel and cost-effective production, testing and packaging process, and also offers the possibility of being packed densely in one and two-dimensional arrays to comply with a plurality of applications such as data communication, sensing applications, and the like. [0003] VCSEL devices inherently operate in a single longitudinal radiation mode due to their short cavity length. On the other hand, typically a plurality of transverse radiation modes are simultaneously present within the resonator, wherein the spectral purity, i.e., the number, width and intensity of wavelengths being present in the output radiation, is substantially defined by the lateral geometry of the VCSEL device. It appears, however, that in certain applications it is important to provide a high spectral purity or a substantially monochromatic emission, such as in spectroscopy applications. A substantially single mode emission, that is, emission of a single longitudinal and transverse radiation mode, is also highly desirable for applications such as positioning, laser printing, or short distance optical interconnections. In a typical VCSEL device having a multimode radiation behavior, an increased transverse mode competition may be observed as the drive current increases, resulting in unstable, non-symmetric, and large divergence angle beam profiles. The multimode behavior entails a plurality of drawbacks in high bit rate data communication systems owing to dispersion effects and also leads to a decrease of spatial and temporal coherence as well as to an increase of noise. [0004] From the plurality of generic VCSEL structures presently known, devices having a selectively oxidized aperture layer have become the subject of intense research and development due to their superior electrical and optical properties compared to alternative structure designs. VCSEL devices having an oxidized aperture layer are available as standard products for a wide range of emission wavelengths. The oxidized aperture layer is typically arranged within the resonator--which is often provided in the form of first and second distributed Bragg reflectors--in the vicinity of the laser active region so as to laterally confine the electric current and the optical field prevailing within the resonator. It turns out, however, that a complex interaction of thermal lensing, spatial hole-burning, self-focusing and non-uniform current injection necessitates the provision of very small oxide apertures in a range of 3-4 micrometers so as to allow an operation of the VCSEL device in a single transverse mode regime, that is, with the emission of the fundamental transverse radiation mode only. The characteristic lateral dimension of approximately 3-4 .mu.m, in turn, causes a large differential resistance, an increased thermal impedance, and a high density of the injected current, which finally reduces the reliability and lifetime of the VCSEL device, especially when moderate output powers are required. Additionally, VCSEL devices of this type may require high operation voltages and may exhibit increased divergence angles. A further drawback of VCSEL devices including very small oxide apertures is their high sensitivity to electrostatic discharge (ESD), which also contributes to a reduced reliability of the device. Moreover, since tight tolerances for manufacturing the apertures are required, production yield may be reduced, thereby rendering these devices less attractive for mass application. [0005] In view of the above-identified problems, it is therefore an object of the present invention to provide a technique that enables to manufacture VCSEL devices having apertures larger than 3-4 .mu.m while still providing a substantially single mode radiation at moderate power levels. [0006] According to one aspect of the present invention, the object is solved by a VCSEL device that comprises a laser active region and a resonator having a first reflector and a second reflector. The first reflector comprises a first plurality of doped layers having alternately a low index of refraction and a high refraction. The first reflector further comprises an aperture layer located above the first plurality of doped layers and formed of an insulating material that is substantially non-transparent for a specified wavelength range, wherein the aperture layer has an aperture with a first characteristic lateral size. The first reflector further includes a second plurality of doped layers having alternately a low index of refraction and a high index of refraction, wherein the second plurality of doped layers has a second characteristic lateral size whereby a difference of the first characteristic lateral size and the second characteristic lateral size is adjusted so as to increase optical losses of the first reflector with respect to higher transverse radiation modes for the specified wavelength range compared to the optical losses created by the aperture layer. Additionally, the VCSEL device comprises a radiation output window that is formed above the first reflector or below the second reflector. [0007] The VCSEL device in accordance with the present invention has a design in which the lateral confinement of the optical field is determined by the interaction of the aperture and the lateral dimension of a portion of the first reflector rather than by an oxide aperture alone. Due to the combined effect of the first and second characteristic lateral sizes the optical losses of the higher order radiation modes are higher compared to the effect of the aperture alone so that the lateral size thereof may be increased compared to conventional approaches without compromising the single mode emission behavior of the VCSEL device. Consequently, the current density through the aperture, and hence through the laser active region, is decreased, thereby significantly increasing the reliability and lifetime of the VCSEL device for a specified required output power. A further advantage resides in the fact that the operating behavior of the VCSEL device is substantially determined by lateral dimensions of the device so that the required device behavior may be adjusted in a simple and cost-efficient manner during manufacturing of the device. Moreover, since the device behavior is significantly influenced and thus in a high degree determined by at least two device dimensions, a deviation of one dimension during the manufacturing process may be compensated for, at least partially, by adjusting the other one of the characteristic lateral sizes in correspondence with the deviation of the first dimension. This allows a higher degree of flexibility and an increased reproducibility of the manufacturing process. [0008] In a further embodiment, the radiation output window has a third characteristic lateral size that is less than the first and the second characteristic lateral sizes. By correspondingly selecting the lateral dimension of the radiation output window, the selective optical conferment of the fundamental transverse radiation mode may still further be enhanced. Moreover, by appropriately selecting the first, second and third characteristic lateral sizes, the far field beam shape may be correspondingly adjusted. [0009] In a further variant, the radiation output window is formed in a metal layer, which may comprise any appropriate material or materials, for example, in the form of two or more sub-layers, to provide for the required optical and electrical characteristics. Advantageously, the metal layer is used as a first electrode to inject current into the laser active region. [0010] In a further embodiment, the first characteristic lateral size is equal to or greater than 5 .mu.m. By correspondingly selecting the size of the aperture in this order of magnitude, the device reliability and lifetime may be increased for a required output power, wherein merely well-established manufacturing techniques are required in fabricating the device. [0011] In a further variant, the first characteristic lateral size is equal to or greater than 6 .mu.m. In this case, a significant enlargement of the aperture is achieved compared to conventional devices so that output powers can exceed 1 mW, while still maintaining the output wavelength variation substantially within a range of a few nanometers. [0012] In a further variant, an absolute amount of the difference of the first characteristic lateral size and the second characteristic lateral size is in the range of approximately 6 .mu.m or less. The above-specified range enables the fabrication of reliable VCSEL devices having a required small output wavelength range at an increased output power even for an aperture size being considerably greater than in conventional devices, wherein fluctuations of the first and/or the second characteristic lateral sizes are substantially non-critical as along as the difference is in the range of 6 .mu.m. In a preferred variant, the difference of the first and second characteristic lateral sizes is in the range of approximately 4 micrometers and less, thereby still further enhancing the transverse mode selectivity. In other illustrative embodiments, the third characteristic lateral size is in the range of approximately 4-7 .mu.m. As previously explained, the lateral dimension of the radiation output window may assist, in combination with a proper selection of the first and second characteristic lateral sizes, in obtaining a superior transverse mode selection and/or in shaping the beam profile in the far field of the laser device. [0013] In a further preferred embodiment, the VCSEL device further comprises a third plurality of doped layers having alternately a low index of refraction and a high index of refraction, wherein the third plurality of doped layers is disposed between the aperture and the second plurality of doped layers. By means of the third plurality of doped layers, a distance between the aperture layer and the second plurality of doped layers, which forms a portion of the first reflector and exhibits the second characteristic lateral size, can be adjusted so as to further enhance the efficiency of suppressing higher-order transverse radiation modes of the aperture and/or the second plurality of doped layers. For instance, depending on the optical characteristics of the doped layers, not more than approximately 7-9 or preferably not more than 5 layers may result in an optimum selectivity efficiency of the aperture layer and the second plurality of doped layers. In a further variant, the aperture layer is formed between the first plurality of doped layers and the third plurality of doped layers so that a distance of the aperture from the active region may be adjusted by appropriately selecting the number of individual layers in the first plurality of doped layers. This enables, in combination with the lateral size of the aperture, to control the efficiency in mode confinement and mode selection of the aperture. For instance, the number of layers of the first plurality may be selected to 9 or less, and more preferably to 5 or less. [0014] In a further embodiment, the second reflector comprises a plurality of doped layers having alternatively a low index of refraction and a high index of refraction. Thus, the second reflector may be designed in a similar fashion as the first reflector, wherein the layers are doped and provided in number so as to obtain the required reflectivity of the resonator formed by the first and second reflectors. [0015] In a further preferred embodiment, the VCSEL device comprises a substrate carrying the second reflector on one surface thereof and further including a metal layer formed on the opposite surface of the substrate. Thus, the metal layer may conveniently be used as a second electrode for generating a current flow through the laser active region. [0016] In a further embodiment, the VCSEL device comprises a contact layer formed between the laser active region and at least a portion of the second reflector, wherein the contact layer is configured to electrically connect the active region to a contact pad. In this embodiment, the provision of a substrate and a corresponding metal layer formed thereon may be rendered obsolete, since driving a current through the laser active region may be accomplished by a top electrode, for instance, in the form of a metal layer having formed therein the radiation output window, and the contact layer in combination with the contact pad acting as a second electrode. Thus, an extremely compact configuration may be obtained when the substrate for forming the VCSEL device is removed after completion of the device. [0017] In one preferred embodiment, the first characteristic lateral size is equal to or less than the second characteristic lateral size. Thus, the aperture acts to substantially define the lateral dimensions of the optical field within the resonator, whereas the second plurality of doped layers having the second characteristic lateral size efficiently suppresses higher order radiation modes owing to increased scattering losses. [0018] In a further preferred embodiment, the first characteristic lateral size is greater than the second characteristic lateral size. In this regime, the aperture defines a gain region, i.e., an effective resonator region, that preferentially supports the fundamental mode with a reduced gain for high order modes, whereas the second plurality of doped layers acts as a mode filter that suppresses the higher order modes in a very efficient manner. [0019] Advantageously, in both regimes, i.e., with the aperture equal to or less, or with the aperture greater than the characteristic lateral size of the second plurality of doped layers, the radiation output window may also act as a mode filter enhancing the modal discrimination. Moreover, when the radiation output window is formed in a metal layer acting as a first electrode, a substantially uniform current injection into the center of the laser active region is accomplished. [0020] In a further variant, the aperture and the second plurality of doped layers have a substantially circular shape and the first and second characteristic lateral sizes represent a first diameter and a characteristic second diameter, respectively. This arrangement results in an output beam having a circular symmetry, thereby achieving simplicity in coupling the output beam into an optical fiber. [0021] In a further variant, the radiation output window has a substantially circular shape to provide for a circular symmetry in the output beam. [0022] In a further embodiment, at least one of the aperture and the second plurality of doped layers has a non-circular shape to provide different optical losses for different polarization states of a low-order radiation mode of the specified wavelength range. Since the lateral asymmetry of the aperture and/or the second plurality of doped layers favors one of two possible polarization states of the fundamental transverse radiation mode, a further improvement in the spectral purity is obtained over a wide range of drive currents. In a further variant the radiation output window may have a non-circular shape to provide for different optical losses for different polarization states of the fundamental transverse radiation mode of the specified wavelength range. The non-circular shape of the radiation output window may be provided in addition to or alternatively to an asymmetry of the aperture and/or the second plurality of doped layers, thereby further enhancing the polarization stability and providing an efficient means for stabilizing a polarization state, respectively. [0023] In another illustrative embodiment a non-circular aperture and/or second plurality of doped layers may be combined with a substantially circular radiation output window so as to maintain a substantially circular beam shape, at least in the vicinity of the radiation output window, while still maintaining a superior behavior with respect to polarization state changes. [0024] In a further embodiment, the VCSEL device comprises a phase matching layer arrange within the resonator, wherein the phase matching layer is configured to laterally pattern the reflectivity of the resonator. Preferably, an optical thickness of the phase matching layer is adapted so as to provide an increased reflectivity at lateral positions with a high probability for the fundamental mode and a low reflectivity at positions of increased amplitudes of the higher-order modes. With the provision of the phase matching layer, which defines a further characteristic lateral dimension regarding the mode confinement and mode selectivity, the performance of the device may further be enhanced and may allow an increased aperture size without significantly compromising the device behavior, Continue reading about Optimizing the increased oxide aperture relative to the laser dimensions... 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